Device for establishing a galvanically separate connection between a telephone line and a signal processing unit at the subscriber end of the telephone line

ABSTRACT

A device for the DC-decoupled connection of a telephone line to a signal processing device at the subscriber end of the telephone line is disclosed. The device exhibits a first circuit which is connected to the telephone line, a second circuit which is connected to the signal processing device, and a transformer which exhibits a first and a second winding, the first winding being connected to the first circuit and the second winding being connected to the second circuit and the first and second winding being DC-decoupled from one another. The device is characterized by the fact that the first circuit exhibits a hybrid circuit for separating the signals of the telephone line into a first signal path which extends from the telephone line to the signal processing device, and into a second signal path which extends from the signal processing device to the telephone line.

This application is based on PCT application No. PCT/DE99/02911.

The invention relates to a device for the DC-decoupled connection of atelephone line to a signal processing device at the subscriber end ofthe telephone line.

From EP-A-0 576 882 a device for the DC-decoupled connection of atelephone line to a signal processing device at the subscriber end of atelephone line is known which exhibits a first circuit which isconnected to the telephone line and which exhibits a second circuitwhich is connected to the signal processing device. In this knowndevice, a hybrid circuit for separating the signals of the telephoneline into a first signal path which extends from the telephone line tothe signal processing device, and into a second signal path whichextends from the signal processing device to the telephone line, isprovided.

The received integrated analog signal of the telephone line is digitizedin the first circuit, multiplexed and transmitted via a firsttransformer to the second circuit where it is de-multiplexed. Thereceived signal of the second circuit is digitized, multiplexed andtransmitted via a second transformer to the first circuit where it isde-multiplexed.

WO 93/39907 A discloses a transformerless data access arrangement fortransferring data between a high-speed modem and a telephone line.

U.S. Pat. No. 5,473,552 discloses an arrangement for isolating acomputer system from a data transmitter network, in which a circuit isfed from a transformer.

WO 96/04715 A discloses a line interface and a method for isolating adata terminal from the line, the signals transmitted via an isolationcircuit being modulated and demodulated and, respectively, mixed.

EP-A-0 661 862 discloses a coupling device for providing forcommunication between a signal processing device and a telephone networkcomprising an infrared transformer and receiver and a sigma/delta coder.

Conventional analog telephone subscriber lines are frequently also usedas data transmission medium. For this purpose, the subscribers are theavailable capabilities of the analog voice telephone network forexchanging data with one another. At the respective subscriber end ofthe telephone subscriber lines, data sources and data sinks which senddata to a remote subscriber or, respectively, receive data from a remotesubscriber are arranged for this purpose. As a rule, the data sourcesand data sinks are conventional computers to which a transmitting deviceand a receiving device in the form of a modem (modulator/demodulator)are connected. The modem and the computer are connected to a powersupply which is independent of the telephone line at the subscriber end.

The rules of the operators of analog telephone networks demand that theterminals connected by a subscriber to the subscriber end of thetwo-wire telephone line remain DC-decoupled from a local earthpotential. With respect to the telephone network, a modem representssuch a terminal which must meet these requirements. A modem requires,therefore, a circuit which, on the one hand, ensures bi-directionalsignal communication via the telephone line and on the other hand,ensures DC-decoupling from the telephone line. Such circuits are alsocalled DATA Access Arrangements (DAA).

Conventional concepts for DC-decoupling are based on transformers, inwhich a winding of the transformer forms the line termination at thesubscriber end of a telephone line. However, transformers have thedisadvantage that they have a large volume relative to other componentsof the modem in the voice frequency range and have a relatively largestatistical variation in their electrical characteristics. A statisticalvariation of the electrical characteristics must be compensated for bycomplex circuits in the modem.

A circuit arrangement for coupling an analog transmission path to adigital transmission path has become known from EP 0 798 885. The analogtransmission path could represent an analog telephone line and thedigital transmission path could represent a digital data bus within amodem. The circuit arrangement provides alternatively capacitors andopto-couplers as DC-decoupling elements. However, the use ofopto-couplers is avoided in many applications because they arerelatively expensive in comparison with the other components. The use ofcapacitors as DC-decoupling elements is advantageous in principle butthe implementation of this concept reveals that a pair of capacitorsmust be provided for each direction of transmission in order to ensurereliable data transmission via the isolation boundary. As a rule,therefore, at least four capacitors (two capacitors for each directionof transmission) are provided in the implementation of the capacitivedecoupling concept. This large number of components is undesirable forproduction reasons. In addition, such a circuit is found to be sensitiveto common-mode interference on the telephone line.

The technical problem of the invention, therefore, consists inspecifying a device for the DC-decoupled connection of a telephone lineto a signal processing device which can be produced inexpensively andcan manage with a small number of components.

The problem is solved by means of a device having the features of claim1. Advantageous embodiments of this device are found in the sub-claims.

The device according to the invention uses a transformer forDC-decoupling. In this arrangement, however, the transformer is notdirectly connected to the telephone line but is only located behind ahybrid circuit which separates the signals of the telephone line into afirst signal path and a second signal path for the respective directionsof transmission. Since the signals of the telephone line can berepresented in a different way, especially in a higher frequency range,behind the hybrid circuit, it is possible to dimension the transformerdifferently i.e., the transformer volume can be made smaller. In such afrequency range, statistical variations of the transformer are scarcelysignificant and do not, therefore, need to be taken into consideration.

In a first preferred illustrative embodiment, the signals of thetelephone line behind the hybrid circuit are supplied to a high-speeddigital/analog converter or, respectively, analog/digital converterbefore the signals thus obtained are supplied to the transformer. In analternative illustrative embodiment, the signals of the telephone lineare demodulated by a radio-frequency demodulator and respectively,modulated by a radio-frequency modulator behind the hybrid circuit.Modulation and demodulation is preferably at a different frequency forthe two directions of transmission.

The circuit section which is DC-coupled to the telephone line preferablyexhibits a power supply device which is alternatively fed from thetelephone line or from the transformer. Feeding from the transformer isespecially advantageous because this can be ensured with relativelysimple means and is not subject to the fluctuations of the power supplyof analog telephone lines.

Further advantages, features and possible applications of the inventionare obtained from the subsequent description of the illustrativeembodiments in conjunction with the diagrammatic drawing in which:

FIG. 1 shows a first illustrative embodiment of the device according tothe invention; and

FIG. 2 shows a second illustrative embodiment of the device according tothe invention.

In the illustrative embodiment of FIG. 1, a transformer 3 which ensuresthe DC-decoupling between the subscriber end of an analog telephone line5 and a signal processing device 6 in the form of a DSP (Digital SignalProcessor) is shown as a DC-decoupling element. Line 4 symbolizes theisolation barrier which extends through the transformer 3 and over whichsignals are exchanged. The transformer 3 has a winding on each side ofthe isolation barrier 4. A primary winding is on the side of thetelephone line 5 and a secondary winding is on the side of the signalprocessing device 6. The windings of the transformer 3 are magneticallycoupled to one another.

The primary winding is connected to a circuit 1. The circuit 1 exhibitsan hybrid circuit 7 which is connected to the subscriber end of theanalog telephone line 5. The hybrid circuit 7 performs atwo-wire/four-wire conversion and provides at the four-wire end a signaloutput and a signal input which correspond to the directions oftransmission between the signal processing device 6 and the remote endof the telephone line 5. In the first direction of transmission from theanalog telephone line 5 to the transformer 3, an analog/digitalconverter 19 which samples the analog signal arriving from the telephoneline 5 at a high sampling rate and outputs the sampled signal values inthe form of digital signals is connected downstream of the hybridcircuit 7. In the reverse direction, a digital/analog converter 20 isprovided which converts the digital signals coming from the signalprocessing device 6 into analog signals in order to output them to thetelephone line 5 via the hybrid circuit 7. The analog/digital converter19 and the digital/analog converter 20 are connected to a digital signalmultiplexer 17 which, in turn, is connected to two winding taps off theprimary winding of the transformer 3. The digital signal multiplexer 17operates in time-division multiplex in both directions of transmission,i.e. it alternately sends signals to the transformer 3 or receivessignals from the transformer 3 in a ping-pong process.

Opposite the circuit 1, a circuit 2 is located on the other side of theisolation barrier 4. This circuit 2 is connected to a transformer 3 onthe one hand and, on the other hand, to a signal processing device 6.For the connection to the transformer 3, a digital signal multiplexer 18which operates in a similar manner to the digital signal multiplexer 17in the circuit 1 is provided in the circuit 2. Accordingly, the digitalsignal multiplexer 18 alternately sends signals to the secondary windingof the transformer 3 and receives signals from the secondary winding ofthe transformer 3 by time-division multiplex in a ping-pong process. Thesending and receiving by the digital signal multiplexer 18 in each casetakes place bit-serially. The serial bit stream to be sent or to bereceived is controlled by a frequency generator 14 which is connected tothe digital signal multiplexer 18. The frequency generator 14, in turn,is connected to a crystal oscillator 15 which is arranged outside thecircuit 2. The circuit 2 also exhibits a digital filter and controlcircuit 16 which is connected between the signal processing device 6 andthe digital signal multiplexer 18. The digital filter and controlcircuit 16 effects pre-processing of the digital data to be sent to thetransformer 3 or, respectively, to be received by the transformer 3. Thedigital filter and control circuit 16 is also clocked by the frequencygenerator 14 for this purpose.

At the end of the analog telephone line 5, a switch 9, for example inthe form of a relay, which is operated by the circuit 1 by means of thedigital signal multiplexer, is provided inside the circuit 1. Thisswitch 9 can also be implemented by a transistor and can be integrated.The switch 9 connects the two a and b wires of the analog telephone line5 to the circuit 8. The switch 9 is closed when the signal processingdevice 6 of the circuit 1 signals via the transformer 3 that a telephoneconnection is to be established. On the other hand, the ringing signalcan be received by the circuit 1 via a separate line. The ringing signalis derived voltage-limited from the protection circuit 30 in order tosignal an incoming request for connection to the circuit 1. The circuit1 then causes the switch 9 to be closed.

The a and b wires of the telephone line 5 are also connected to arectifier and charge regulating circuit 8 which is also controlled bythe circuit 1. The circuit 8 rectifies the signals on the telephone line5, on the one hand, in order to regulate the current through the a and bwires of the telephone line 5 in accordance with the rules of therespective network operator of the telephone line 5, on the other hand,in order to signal an off-hook or on-hook condition.

The circuit 1 receives its supply voltage from two winding taps on theprimary winding of the transformer 3. These winding taps are connectedvia the diodes 12 and a capacitor 13 to an independent referencepotential 11 of the circuit 1. The junction between diode 12, diode 31and capacitor 13 is connected to a voltage regulating unit 10 in thecircuit 1 which provides a regulated voltage to the remaining parts ofthe circuit 1. Alternatively, the voltage regulating unit 10 could beconnected to the telephone line 5. Many network operators provide alimited power supply for the terminal connected to the subscriber line.

The transformer 3 ensures DC-decoupling between the telephone line 5 andthe signal processing device 6. The special feature of the inventionconsists in that the transformer 3 is not directly connected to the aand b wires of the telephone line 5 but that the hybrid circuit 7 isinterposed between the telephone line 5 and the transformer 3. Theexchange of signals over the isolation barrier 4 takes place by means ofhigh-speed bit streams in a ping-pong process. Due to the high frequencyof the bit streams, a radio-frequency transformer can be used. Therequirements for this transformer with respect to linearity and phasedistortion can be lower than is the case with the known low-frequencytransformers which are connected directly to the telephone line 5.

The circuit has its own reference potential 11 which is independent of areference potential in the circuit 2 or in the signal processing device6. The circuits 1 and 2 are preferably in each case integrated on onesemiconductor chip. The rectifier and current regulating circuit 8, theprotection circuit 30, the diodes 12, 31 and the capacitor 13 and partsof the circuit 10 are preferably discrete components which are connectedto the circuit 1 designed as an integrated semiconductor circuit.Similarly, the crystal oscillator is a discrete component 15 which isconnected to the circuit 2.

In addition, the digital signal multiplexer 17 in the circuit 1 exhibitsa clock regeneration circuit in order to recover the clock generated bythe frequency generator 14 in the circuit 2 and contained in thetransmitted bit stream and provide it to the circuit sections there,especially the digital signal multiplexer 17, the analog/digitalconverter 19 and the digital/analog converter 20. At the same time, therectified data clock can be used as power source by the circuit 1.Diodes 12, 31, capacitor 13 and the voltage regulating unit 10 areprovided for this purpose.

FIG. 2 shows a second illustrative embodiment of the invention. Elementsfulfilling a similar function to elements in the illustrative embodimentof FIG. 1 are in each case designated by the same reference symbols inFIGS. 1 and 2. Reference is, therefore, made to the discussions relatingto the illustrative embodiment of FIG. 1 to obtain a more detailedexplanation. In concept, the illustrative embodiment of FIG. 2 differsfrom that of FIG. 1 in that, instead of high-speed bit streams, analogradio-frequency signals are transmitted via the transformer 3. For thispurpose, a modulator 23 which modulates the signal coming from thehybrid circuit 7 onto a radio-frequency carrier with the frequency RF1is provided in the circuit 1. The carrier signal thus modulated issupplied to an analog signal mixer 21. In the reverse signal direction,a demodulator 25 which is driven with a radio-frequency carrier signalhaving the frequency RF2 is provided between the analog signal mixer 21and the hybrid circuit 7. The analog signal mixer 21 sends the signalcoming from the modulator 23 to the transformer 3 and receives thesignal to be sent to the demodulator 25 from the transformer 3. In thepreferred illustrative embodiment, the carrier frequencies RF1 and RF2are spaced apart sufficiently so that there will be no beating of themodulated signals. In the analog signal mixer 21, the respectivedirections of transmission are separated from one another by transmitfilters and receive filters.

Similarly, an analog signal mixer 22 which also exhibits a transmitfilter and a receive filter is provided in the circuit 2 of FIG. 2. Thesignal received from the transformer 3, which has been modulated withthe carrier frequency RF1 in the modulator 23, is demodulated with theaid of a demodulator 26 which is also operating at the carrier frequencyRF1 in the circuit 2. In the transmit direction, a modulator 24 isprovided which operates at the carrier frequency RF2. The output signalof the modulator 24 is supplied to the transformer 3 via the analogsignal mixer 22. The analog signal mixer 21 in the circuit 1 receivesthis signal and supplies it to the demodulator 25 which demodulates itat the same carrier frequency RF2.

At the output of the demodulator 26, an analog/digital converter 27which converts the received analog signal into a digital signal andsupplies it to a digital filter and control circuit 29 is provided inthe circuit 2. In the reverse direction, digital signals from thedigital filter and control circuit 29 are supplied to a digital/analogconverter 28 which supplies an analog transmit signal to the modulator24.

In the illustrative embodiment of FIG. 2, a frequency-division multiplexmethod is used instead of the time-division multiplex method of FIG. 1for transmitting bi-directional signals via the transformer 3. Sinceradio-frequency signals are transmitted via the transformer 3, the samerequirements apply to the transformer 3 of FIG. 2 as for the transformer3 of FIG. 1. In the illustrative embodiment of FIG. 2, the hybridcircuit 7 is connected between the transformer 3 and the telephone line5 to implement the frequency-division multiplex method. The circuit 1obtains its voltage supply from special taps on the primary winding ofthe transformer 3 via diodes 12, 31 and a capacitor 13. As in theillustrative embodiment of FIG. 1, the switch 9 is controlled completelywithin the circuit 1.

Both illustrative embodiments combine the advantages of low costs and alower form factor compared with previous solutions. Since the circuit 1is supplied with power via the transformer 3, nonetwork-operator-oriented design of the circuit 1 is required. Thecircuit 1 is, therefore, also insensitive to interference andfluctuations of the direct voltage on the a and b wires of the telephoneline 5. The switch 9 is preferably controlled in the circuit 1. Thecaller ID contained in an incoming request for connection can bedetermined by programming the digital filter and control circuit 16 and29, respectively, in circuit 2 and transmitted to the signal processingdevice 6. Any interfering voltages which may be present on the analogtelephone line 5 are suppressed by the proposed transmission techniques.As a result, a very high data rate can be achieved over the analogtelephone line 5.

1. Device for the DC-decoupled connection of a telephone line to adigital signal processing device at the subscriber end of the telephoneline, comprising a first circuit, which is connected to the telephoneline; a transformer; a second circuit, which is connected to the digitalsignal processing device; the first circuit exhibiting a hybrid circuitbetween the transformer and the telephone line for separating thesignals of the telephone line in a first signal path which extends fromthe telephone line to the digital signal processing device, and into asecond signal path which extends from the digital signal processingdevice to the telephone line; the transformer having a first and asecond winding; the first winding being connected to the first circuitand the second winding being connected to the second circuit; the firstand second winding being DC-coupled from one another; the first andsecond circuits transmitting the signals of the first and second signalpaths transmitting bi-directionally by a ping-pong process decoupled viathe transformer for both directions of transmission.
 2. Device accordingto claim 1, characterized in that the first circuit in the first signalpath exhibits an analog/digital converter which follows the hybridcircuit, and the first circuit in the second signal path exhibits adigital/analog converter which precedes the hybrid circuit.
 3. Deviceaccording to claim 2, characterized in that the output of theanalog/digital converter and the input of the digital/analog converterare connected to a first digital signal multiplexer which, in turn, isconnected to the first winding of the transformer.
 4. Device accordingto claim 3, characterized in that the signal multiplexer is operated insuch a manner that the first and the second signal path are alternatelyconnected to the transformer.
 5. Device according to claim 4,characterized in that the second circuit exhibits a second digitalsignal multiplexer which is connected to the second winding of thetransformer.
 6. Device according to claim 5, characterized in that thesecond circuit exhibits an oscillator circuit which provides the clockfor the second digital signal multiplexer.
 7. Device according to claim6, characterized in that the first digital signal multiplexer exhibits aclock recovery circuit which recovers the clock of the oscillatorcircuit and provides it to the first circuit.